1. Field of the Invention
The present invention relates to lasers. More particularly, the present invention is related to a solid-state laser resonator having a pair of reflectors and a bar-like laser medium interposed between the pair of reflectors.
2. Background of the Invention
A conventional example will be first described with reference to FIG. 1. There is shown a cylindrical solid-state laser medium 6, for example Nd:YAG. In addition, a pair of flat mirrors 4F and 5F are provided at opposite ends of the laser medium 6 so that the surfaces of the mirrors are opposed to each other.
A plurality of laser diodes 1 are also shown. In this example, four such diodes 1 are used. Each of the laser diodes 1 emits a laser beam having a wavelength of, for example, 0.81 .mu.m. The laser beam from the laser diodes 1 are individually incident to the end faces of a set of optical fibers 2. The other set of end faces 2T of these four optical fibers 2 are fixed in place in an overcoating 2B locating them at the four vertexes of, for example, a 400- .mu.m square as illustrated. The four laser beams are divergently excited from the other end faces of the four optical fibers 2 which are fixed at the vertexes of the square. Then, the laser beams exiting the end faces 2T are incident to coupling lens 3, which is like a convex lens, of which the magnifying power is, for example, 2. The four laser beams exit from the lens 3 as pumping laser beams 8 and are incident to the back of the flat mirror 4F.
The flat mirror 4F has a transparent base material coated with a mirror coating layer. The mirror coating layer is deposited on the inner side of the transparent base material so that 100% of the above mentioned pumping laser beams 8 are transmitted therethrough and 100% of the incident laser beam, which for example has a wavelength of 1.06 .mu.m, is reflected. The other mirror 5F also has a transparent base material and a mirror coating layer. The mirror coating layer is formed on the inner side of the transparent base material so that it reflects, for example, 95% of incident 1.06 .mu.m laser beam. The remaining 5% is transmitted through the mirror 5F as an output beam.
Therefore, the four pumping laser beams 8 which are incident to the back of the flat mirror 4F arrive at the end face of the laser medium 6 and are focused within the laser medium 6. The laser beams thus heat four excited regions 7 within the laser medium 6, so that thermal lenses, which are convex lenses, are formed therein. This results in creating four resonator portions which are formed of a pair of flat mirrors 4F, 5F, the laser medium 6 therebetween and the four thermal lenses within the laser medium. Thus, a pair of flat mirrors 4F, 5F has four pairs of flat mirror portions for the four resonator portions. Referring to FIG. 2, the points of X=1 mm, Y=1 mm is represented by A1, the point of X=-1 mm, Y=-1 mm by A2, the point X=0 mm, Y=1 mm by B1, and the point of X=1 mm, Y=0 mm by B2. FIG. 3 shows the temperature distributions on the cross-sections O-A1 and B1-B2. From the graph of temperature distributions, note that the temperature distribution on the cross-section B1-B2 is symmetrical with respect to the straight line passing through the center (X=0.5 mm, Y=0.5 mm) of the excited region 7. Also note that the temperature distribution on the cross-section O-A1 is asymmetrical therewith. In addition, the optical axis of the thermal lens is deviated from the axis of the excited region 7 of the laser medium.
Therefore, the conventional solid-state laser resonator is reduced in its oscillation efficiency by an amount corresponding to the deviation of the optical axes of the thermal lenses formed within the laser medium 6, or by the amount that the excited regions and the oscillation regions are separated. Also, the asymmetric property of each of the four excited regions 7 of the laser medium causes an aberration in the output laser beam.